1. Field of the Invention
The present invention relates to an apparatus and method for detecting a wobble signal read from an optical disc such as a writable compact disc (CD) or digital versatile disc (DVD).
2. Description of the Related Art
It is common that a writable optical disc, such as a digital versatile disc-random access memory (DVD-RAM) or digital versatile disc-re-writable (DVD-RW), has grooves formed along spiral or concentric tracks. Here, portions of the optical disc other than the grooves are typically called lands. Data can be recorded on only any one or both of each groove and each land according to a writing method. A specific variation is applied to a wall of each groove in a groove formation process, and a specific frequency signal is generated based on the specific variation in a recording/reproduction process, so it can be used as auxiliary clock means. Here, the specific variation is called a wobble and the specific frequency signal is called a wobble signal.
FIG. 1 shows the construction of a conventional apparatus for detecting a wobble signal read from an optical disc. As shown in this drawing, the conventional wobble signal detection apparatus comprises a band pass filter (BPF) 10 for filtering a push-pull signal, or an analog wobble signal, read from a writable optical disc, for example, a DVD-RW at a predetermined frequency band to remove a high-frequency noise component, a direct current (DC) offset component, etc. therefrom. An analog/digital (A/D) converter (ADC) 11 is provided to A/D-convert an output analog wobble signal of the predetermined frequency band from the band pass filter 10 to output a digital wobble signal. A wobble signal detector 12 acts to slice the A/D-converted digital wobble signal on the basis of a predetermined reference level, for example, a zero level to detect/output a square-wave wobble signal. A wobble phase locked loop (PLL) 100 is provided to output a wobble PLL clock synchronized with the square-wave wobble signal. FIG. 2 shows measured waveforms of the filtered wobble signal from the band pass filter 10, the sliced wobble signal from the wobble signal detector 12 and the PLL clock from the wobble phase locked loop 100.
The conventional wobble signal detection apparatus further comprises a bit detector 16 for detecting/converting the square-wave wobble signal into a stream of bits having values of 1 or 0, using the wobble PLL clock. A synchronous (Sync) detector 17 acts to detect a synchronous pattern placed in the square-wave wobble signal from the bit stream and generate and output a synchronous signal corresponding to the detected synchronous pattern. An address decoder 18 is provided to decode a physical address of the optical disc from the bit stream on the basis of the synchronous signal.
The wobble PLL 100 includes a phase error detector 13 for detecting a phase error at a point of time that the A/D-converted wobble signal crosses a zero point from positive to negative, namely, a negative zero crossing point (referred to hereinafter as ‘NZCP’) as shown in FIG. 3. A time count value that determines an oscillating frequency of a digital controlled oscillator (DCO) 15 in the wobble PLL 100, for example, a free down time count value, is always corrected with the phase error detected at the NZCP.
At this time, in a case (lead case) where the phase of the PLL clock, which is generated by the digital controlled oscillator 15, is ahead of that of the A/D-converted wobble signal, the phase error detector 13 detects/generates a positive phase error as shown in FIG. 3 and outputs the generated phase error to a loop filter 14 in the wobble PLL 100. The loop filter 14 then corrects a time count value for determination of a clock frequency of the digital controlled oscillator 15 into a smaller value according to the positive phase error.
On the other hand, in a case (lag case) where the phase of the PLL clock generated by the digital controlled oscillator 15 is behind that of the A/D-converted wobble signal, the phase error detector 13 detects/generates a negative phase error as shown in FIG. 3 and outputs the generated phase error to the loop filter 14, which then corrects the time count for determination of the clock frequency of the digital controlled oscillator 15 into a larger value according to the negative phase error.
As a result, the wobble PLL 100 continuously performs a phase error correction operation for synchronization of the PLL clock with the wobble signal by detecting a phase error at the NZCP and correcting the time count for determination of the clock frequency of the digital controlled oscillator 15 on the basis of the detected phase error.
However, recently, as the optical disc becomes higher in recording density, recording tracks thereof have denser pitches, resulting in a greater crosstalk effect caused by a wobble signal of an adjacent track. This greater crosstalk effect makes the output wobble signal from the band pass filter 10 very small in signal to noise (S/N) ratio, so a stable wobble signal cannot be detected by the conventional wobble signal detection apparatus.
In addition, provided that a tracking servo and focusing servo are unstable or a surface vibration of the optical disc, etc. occur, a low-frequency fluctuation component and DC offset component will be introduced into the output wobble signal from the band pass filter 10 or the wobble signal will vary in amplitude. For example, as shown in FIG. 4, in the case {circle around (1)} where a stable wobble signal with a constant amplitude is outputted from the band pass filter 10, a stable wobble signal {circle around (4)} is normally detected by the wobble signal detector 12. Alternatively, in the case {circle around (2)} where a low-frequency fluctuation component and DC offset component are contained in the output wobble signal from the band pass filter 10, or in the case {circle around (3)} where the output wobble signal from the band pass filter 10 has a varying amplitude and DC offset component, an abnormal wobble signal {circle around (5)} or {circle around (6)} is detected by the wobble signal detector 12. Consequently, the wobble phase locked loop cannot normally perform the phase error correction operation.